1. Field of the Invention
The present invention relates to metal plating of substrates, and more particularly but not by way of limitation to an improved aqueous acidic plating solution for the metal plating of iron-containing substrates to alter the existing surface properties or dimensions of such substrates. In one aspect the present invention relates to an improved method for metal plating iron-containing substrates.
2. Discussion of the Prior Art
Numerous processes have heretofore been employed for depositing metallic coatings on a substrate to alter the existing surface properties or the dimensions of the substrate. Typical of such processes are the electroplating process, the electroless plating process and the displacement plating process.
The electroplating process consists of connecting the parts to be plated to the negative terminal of a direct-current source, another piece of metal to the positive terminal and immersing both in a solution containing ions of the metal to be deposited on the surface of the parts connected to the negative terminal. Generally, the metal connected to the positive terminal (i.e. the anode) is the same material that is to be plated on the parts connected to the negative terminal (i.e. the cathode). Thus, metal dissolves at the anode and is plated at the cathode.
Most plating solutions utilized in the electroplating process are of the aqueous type. However, there is limited use of fused salts or organic liquids as solvents. Non-aqueous solutions are employed for the deposition of metal with lower hydrogen over voltage, i.e. hydrogen rather than the metal is reduced at the cathode in the presence of water.
Electroless plating and placement plating are two other deposition processes that are closely related to electroplating. Displacement plating occurs when the metal deposited is more noble than the substrate and the substrate dissolves. The reaction ceases when the substrate is completely covered by a pore-free deposit.
The electroless plating process is a chemical reduction process which, once initiated, is autocatalytic. The process is similar to electroplating except that no outside current is needed. The metal ions are reduced by chemical agents in the plating solutions, and deposit on the substrate. Electroless plating has found application for depositing a metal on a metallic substrate which is an irregularly shaped object but requires a uniform coating. Thus, an advantage of electroless plating over plating with current (i.e. electroplating) is that a more uniform thickness of the coating can be obtained using the electroless process than can be achieved with the electroplating process, especially when the objects have an irregular shape of deeply recessed areas which are difficult to plate.
While each of the before-mentioned prior art processes have been widely used in the depositing of metallic coatings on a substrate to alter the existing surface properties and dimensions of the substrate, improvements are constantly being sought which will enable one to more readily carry out the desired plating process and provide a more controlled and uniform thickness of the surface coating. Further, it would be highly desirable if improved plating solutions could be formulated which are substantially non-toxic in nature, possess safe handling characteristics, and which can readily be disposed of without deleterious effects to the environment.